MEMBRANES PREPARED FROM CROSSLINKABLE SOLUBLE POLYMERS

Abstract: A process is disclosed for the formation of a composite membrane having a discriminating layer affixed to a porous support layer. The process comprises the formation of the discriminating layer by irradiating an aqueous polymer solution containing from 0.01 to 30 weight percent polymer under conditions which are sufficient to form a polymer film on the surface of the polymer solution at the interphase of the solution and a blanketing fluid. A preferred polymer is a hydroxyethylmethacrylate/vinylbenzyldimethyl sulfonium/methacrylic acid terpolymer.

Description: MEMBRANES PREPARED FROM CROSSLINKABLE SOLUBLE POLYMERS The present invention is related to composite membranes useful in fluid separations and methods for their preparation. Semi-permeable composite membranes prepared from various synthetic polymeric compositions are used in various commercial and industrial applications for the separation of various components found in liquids or gases. Reverse osmosis and nanofiltration membranes are typically relatively thin in order to provide a desirable, i.e., relatively high, flux rate. Thus, it is generally necessary that the reverse osmosis or nanofiltration membrane be laminated onto a porous support material. This support material will generally possess cnaracteristics which make it desirable for such a use. Such characteristics include a sufficient number of pores large enough to permit water or other permeates to pass through the support without adversely affecting the flux rate or separation efficiency of the entire composite membrane. Conversely, the pore size should not be so large that the membrane tends to be forced into the pores or rupture during use. The present invention is directed to a composite membrane which comprises a polymeric discriminating layer affixed to a porous support layer. The discriminating layer is formed by irradiating a substantially aqueous polymer solution under conditions which are sufficient to form a polymer film on the surface of the polymer solution at the interphase of the solution and a blanketing fluid which is immiscible with the polymer solution. A significant feature of the present invention is that it is not necessary to dry the polymer solution prior to irradiation, provided the bulk concentration of the polymer in solution is at an appropriate level to form the film upon irradiation. The membranes of the present invention can be made to exhibit a variety of molecular weight cut offs (MWCOs) by altering the process for the preparation of the membrane rather than by altering the polymer backbone of the discriminating layer. The membranes 10 made by the process of the present invention preferably have MWCOs ranging from 150 to 2000 daltons. Further, the present invention largely avoids the use of organic solvents. Additionally, the 15 membranes of the present invention are resistant to degradation by chlorine and other oxidizers. The discriminating layer of the membrane of the current invention is prepared by irradiating a 20 substantially aqueous polymer solution under conditions such that a thin film is formed on the surface of the polymer solution at the interphase of the solution and a blanketing fluid which is immiscible with the polymer 2c - solution. Typically, the polymer solution is open to the air and the film is formed at the air/solution interphase. However, the blanketing fluid may be an atmosphere other than air or it may be a liquid which is transparent to radiation and which does not interfere 30 with the formation of the discriminating layer. When irradiated, the polymer solution from which the discriminating layer is formed is - 3t - substantially aqueous. That is, it need not have been subjected to a drying step and is not dry or in the form of a gel. The primary requirement is that the bulk concentration of the polymer in solution is at an appropriate level to form the film upon irradiation. The appropriate level is from 0.01 to 30 weight percent polymer. The ability to form the film in the absence of drying is normally of particular advantage in an industrial setting. As noted above, the polymer solution is substantially aqueous. By "aqueous", it is meant that the solvent is typically water although water compatible co-solvents may by used in conjunction with the water. Water is generally preferred as a solvent due to cost and availability and should constitute a minimum of fifteen percent of the solution. At least fifty percent water is preferred and at least seventy percent water is more preferred. Compatible co-solvents include ethylene glycol, lower alkanols and similar substances. The primary restriction on co-solvents is that they do not interfere with film formation. Additionally, the polymer solution optionally contains various additives including certain salts and acids. Specific examples of additives include NaCl, H 2 S0n, H3PO4, CH3COOH, HNO3, LiCl, MgCl2, NaS0i+, Na, HPO4, and HC1. Additionally, photosensitizers such as the sodium salt of 2-naphthalenesulfonic acid may be used. Depending on the desired properties of the composite membrane, additives can be used to enhance flux and/or selectivity. The discriminating layer is formed at the surface of the aqueous polymer solution and may then be recovered and affixed to an appropriate support. Alternatively, the aqueous polymer solution may be applied to a support and then irradiated to form the discriminating layer in situ . It is a particular advantage in an industrial setting that the discriminating layer can be formed and affixed to the porous support in a single process which may be continuous. In one preferred embodiment, the membrane of the present invention is prepared in a process comprising the following steps: (1) contacting a support with at least one polymer solution which is from 0.01 to 30 weight percent polymer in a substantially aqueous solvent; (2) irradiating the polymer solution under conditions such that a discriminating layer is formed at the blanketing fluid/solution interphase; and (3) affixing the discriminating layer to the support. In addition to these essential steps which may be performed simultaneously or sequentially, the process may comprise additional steps. In the preparation of the membrane, it is desired that the support be "wet" and that the discriminating layer be adhered to the support sufficiently to prohibit unrestrained swelling of the discriminating layer. Depending on the particular support and discriminating layer used, this may be accomplished by coating a support with a single polymer solution that performs all the desired functions. Alternatively, the support may be coated with different solutions prior to irradiation. For example, it may be desirable to treat a support with a wetting solution; then with a polymer solution that forms the basis for an affixing layer which, at a minimum, functions to adhere the discriminating layer to the support sufficiently to prohibit unrestrained swelling of the discriminating layer: and then with a solution from which the discriminating layer is formed. Depending on the support and polymer solutions used, the 'wetting' solution, the 'affixing' solution 10 and the discriminating layer solution may be the same polymer solution and may be applied in a single layer or in multiple layers. Alternatively, the solutions may use the same polymer, but use a different solvent or a 15 different concentration. Alternatively two or more different polymer solutions may be used. In one alternative embodiment, the process comprises the following steps performed simultaneously or sequentially: 20 (1) contacting a support with a wetting solution; (2) contacting the wet support with an affixing ? layer forming polymer solution which is from 0.01 to 10 weight percent polymer; (3) contacting the wet support with a discriminating layer forming polymer 30 solution which is from 0.01 to 30 weight percent polymer in a substantially aqueous solvent; (4) irradiating the discriminating layer J forming polymer solution under conditions such that a discriminating layer is formed at the blanketing fluid/solution interphase; and (5) affixing the discriminating layer to the support. In another alternative embodiment, the process comprises the following steps performed simultaneously or sequentially: ( 1 ) contacting a support with an affixing layer forming polymer solution which is from 0.01 to 10 weight percent polymer; (2) contacting the support with a discriminating layer forming polymer solution which is from 0.01 to 30 weight percent polymer in a substantially aqueous solvent; (3) irradiating the discriminating layer forming polymer solution under conditions such that a discriminating layer is formed at the blanketing fluid/solution interphase; and (4) affixing the discriminating layer to the support. In each embodiment, the process may be continuous. Polymer solutions and wetting solutions may be applied to the support by techniques known to one skilled in the art. Conventional techniques include adsorption, dipping, casting, spraying, wiping, rolling, or filtration of the coating solution through the substrate. Excess coating may be removed by draining or drawing a smooth instrument such as a blade or roller across the surface. The temperature of the coating solutions are selected so as to avoid conditions detrimental to the resulting membrane. Other than as discussed herein, operating parameters for applying the polymer solutions are not critical so long as the resulting membrane is not deleteriously affected. The process may be conducted at temperatures ranging from 0° to 55°C. Ambient temperatures, i.e., 10° to <45°C are 10 generally convenient and therefore preferred. The support is typically porous and does not significantly impede the transport of fluids across the membrane as compared to the discriminating layer. It is m e used to provide mechanical strength to the membrane. Examples of suitable supports include a microporous polymer such as polysulfone, polyethersulfone, polycarbonate, polyvinylidene chloride, Nylon, polyetherether ketone, polybenzimidazole, cellulose 0 acetate or other cellulose esters. The manner in which the discriminating layer is affixed to the support is not critical to the present invention so long as the resulting membrane has the 25 desired characteristics. The discriminating layer may be affixed through chemical or physical means. Methods, known to those skilled in the art, may be used such as drying or acid catalyzed condensation. Activation of a thermally sensitive crosslinking agent is a suitable 30 method. This may be accomplished in a separate step, subsequent to the formation of the discriminating layer, in which the membrane is subjected to elevated temperatures, in for example, an oven in a temperature ^ 5 range from 50°C to 200°C, more preferably from 75°C to 150°C. In an alternative embodiment, the discriminating layer maybe affixed to the support as a result of heat that is incidental to the irradiation to form the discriminating layer. The polymer used in the discriminating layer polymer solution must be capable of film formation upon exposure to radiation. The polymer must also be surface active in that it is necessary that the surface concentration of the polymer in the discriminating layer polymer solution be sufficient to form a surface film. Multi-component polymers, such as those useful in the present invention, usually consist of different mono eric units each of which contributes a desired characteristic to the resulting polymer and ultimately to the finished membrane. In order to impart the desired properties to the membrane discriminating layer, polymeric reactants may contain groups in the repeating unit in addition to a moiety directly bearing or including a reactive cationic or nucleophilic group, provided these groups do not adversely affect the membrane or its formation. For example, in cationic vinyl addition polymers, such methacrylate derivatives as — to C 3 al ylene)—OH alkyl or alkenyl) wherein m is an integer from 1 to 20, may be present to advantage in membranes for reverse osmosis. Photosensitivity in the polymer is preferably obtained by the presence of onium groups on the polymer backbone. Known onium groups include aryl cationic moieties, which have been described as photoacid generating initiators in the prior art. For example, The Chemistry of the Sulfonium Group, edited by C. J. M. Stirling and S. Patai, pp. 107-122, John Wiley & Sons (1981), describes the photochemistry of sulfonium compounds. Advances in Polymer Science, 62, pp. 1-48, Springer-Verlag Berlin, Heidleberg (1984), describes the cationic polymerization using iodonium or sulfonium salt photoinitiators. It has been found that in preferred embodiments, the polymer bearing a plurality of photolabile onium groups will react at ambient temperatures with even weakly reactive nucleophile groups, such as amides, urea moieties or sulfonic acid salts. Preferred photolabile onium groups include sulfonium, quaternary ammonium, phosphonium, pyridinium, thiazolinium, imidazolinium or azetidinium groups. Diazonium groups are not onium groups as the term is used herein. Techniques and processes for making compounds bearing the desired moieties are well known in the prior art. U.S. Patents 2,676,166; 2,891,025; 3,269,991; 3,329,560; 3,429,839; 3,544,499; 3,636,052; 3,723,386; 3,962,165; 4,002,586; 3,804,797; 4,337,185, 4,483,073; 4,426,489; 4,444,977 and 4,477,640 are incorporated herein by reference to illustrate techniques for making such compounds. Especially preferred as photolabile oniums are those containing a sulfonium, quaternary ammonium or phosphonium group. Preferably, the substituents on the photolabile onium are each independently hydroxyalkyl, phenyl or alkyl groups or are heterocyclic saturated moieties which include the onium in the ring. Most preferably, the photolabile onium group is bonded to the -(-CH2-)- moiety of a benzyl group and is a dialkyl sulfonium, trialkyl phosphonium or trialkyl ammonium moiety wherein each alkyl has from about 1 to about 16 carbon atoms or is a sulfonium, alkyl phosphonium or alkyl ammonium where two of the valences are part of a five- or six-member ring including the onium. The chromophore group is preferably an aromatic group. The chromophore group may be joined to the onium moiety by a linking group (chromophore-linking group- onium) advantageously selected from methylene, i.e. , (-CH2-) > ethylidene (i.e., Especially preferred as a chromophore is a phenyl group which is pendant from a polymer backbone. Especially preferred as a linking group is methylene or OH -0CH 2 CHCH 2 - Preferably, the benzyl onium salt groups are part of a a vinyl addition polymer. Such polymers can readily be prepared by conventional vinyl addition polymerization of vinyl benzyl chloride with other compatible monomers followed by reaction of the benzyl chloride with a suitable onium precursor. For example, dialkyl sulfide will react with the benzyl chloride group pendant from a vinyl addition polymer to form a dialkyl sulfonium group. Tertiary amines or phosphines will react with benzyl chloride in a similar manner. Alternatively, a polystyrene or styrene copolymer can be chloromethylated via conventional techniques to introduce benzyl chloride groups. The benzyl chloride groups can then be converted to onium groups as described hereinbefore for the vinyl benzyl chloride polymers. T e anion associated with the photolabile onium group is advantageously selected so as to promote reaction between the photolabile onium group and the nucleophile group present, when exposed to radiation. Any anion is operable so long as the reaction is not deleteriously affected. Optionally, inner salts or partial inner salts of onium compounds can be employed, such as a polymer bearing both carboxylate and photolabile onium groups. Some anions, such as hydroxide, in some embodiments will also make a sulfonium or certain other onium groups more susceptible to the competing thermal reaction or degradation. The counterion can be readily changed by contacting the compound bearing the onium group with an appropriate ion exchange resin in the conventional manner to effect conversion to the desired anion. Preferred polymers are those which provide sufficient polymer concentration at the interphase with the blanketing fluid to permit thin film formation. A preferred class of photoreactive systems is represented by Formula I where illustrative embodiments of ArC, Z, Q e and PNu θ are presented in Table A. It should be noted that the positive charge on Q ® and negative charge on PNa θ may be one or greater with the proviso that in each instance they are equal so that the overall charge is neutral. TABLE A In Table A, each moiety at each occurrence is independently selected from the group consisting of R' = a polymer or copolymer backbone optionally inertly substituted or bearing a plurality of Q ® and/or PNu θ ; R- | and R2 are each independently hydrogen c 1"" c 1β alkvl > or -CH 2 (CH ) u 0H, preferably CH3 or tertiary-butyl, where u = 1 to 12; Rp is a fluorinated alkyl. Rp may be an alkyl which is not fully fluorinated, but no more than one atom of hydrogen or chlorine should be present in place of fluorine for each carbon atom. Rp is preferably ^^CF^, where v is an integer from 1 to 12, more preferably from 6 to 12, or Rp is preferably -(CH 2 -)- χ -(-CF2)F, where x is an integer 1 or 2 and y is an integer from 1 to 12, more preferably from 6 to 12; and R = C- ] to C1Q alkyl, phenyl or a polymer or copolymer which is optionally inertly substituted or bears a plurality of Q ® and/or PNu θ . The photoreactive moiety, ArC-Z-Q ® , may be used as a low molecular weight species, for example The photoreactive moiety is preferably attached to a polymer, either as a pendant group or as an end group. For example, a class of polymers can be represented by Formula II: A and E are each terminal groups resulting from a vinyl polymerization, and B, C and D are internal covalently bonded groups which can be arranged in any sequence. The subscripts m, n and o represent molar ratios and m+n+o = 1.00 where m is in the range from about 0.03 to about 1.00, n is in the range from 0 to 0.97 and o is in the range from 0 to 0.96. The subscript p is the average degree of polymerization, preferably from about: 2 to 1,000, more preferably from about 100 to about 1000. In Formula II, B is a photoreactive moiety which has the formula R^-Y-ArC-Z-Q ® wherein R^ is a group which includes a carbon-carbon single bond formed during vinyl addition polymerization of the polymer and Y is a chemical bond or a noninterfering, bivalent moiety. ArC is a chromophore, Z is a linking group and Q ® a photolabile onium as defined hereinbefore. Preferably, R^ is the residue of an ethylenically unsaturated monomer, more preferably fCH 2 -CH τ or CH £CH 2 -C} and Y is a chemical bond (in which case B is R Q -ArC-Z-Q ® ) or a noninterfering connecting group, such as 0 fl -(-CH 2 CH-0-)- u -C-0- CH 2 CH0-)- u CH 2 CH 2 0^ u and CH wherein u is independently at each occurrence an integer from 1 to 20 and v is an integer from 1 to 12, but preferably 1. Illustrative examples of B include CH3 -CH 2 -C- C=0 In Formula II, "C" is a group derived from an ethylenically unsaturated monomer which has the formula: RP-Y-PNu© where RP is a residue of an ethylenically unsaturated monomer, PNu© (as defined hereinbefore), and Y' is a chemical bond, in which case "C" is RP-PNu© , or Y' is a noninterfering group, such as -C(0)-(CH 2 ) u or -C(0)0(CH 2 u wherein u is an integer from 1 to 20. Illustrative of tip 11 are In Formula II D has the formula 0 -R h -G ' where R h is an organic group and the residue from a polymerized ethylenically unsaturated monomer, more preferably £CH 2 -CHj or tCH 2 -C(CH 3 )J 0 and G is an organic noninterfering group, such as 5 0CH 2 CH 2 R F ° -C-O-R or -C(0CH 2 CH 2 -0R" wherein R is as previously defined for Table A, Rp is as previously defined for Table A, R" is a C- | to C- | g alkyl or aralkyl, and u is an integer from 1 to 20 and v is an integer from 1 to 40. A and E in Formula II are each independently end groups consistent with vinyl addition polymerization. Illustrative end groups are CH (CH 2 )S-, H-, CH 3 " , (CH 3 ) 3 CO-, Cl- and -OH. In another embodiment of the subject invention, the first and second compounds can be prepared in situ from polymerizable moieties bearing at least one nucleophilic or onium group. For example, vinylbenzyl chloride, hydroxyethylmethacrylate and methacrylic acid can be copolymerized using a free radical initiator. In general, after the polymers bearing onium groups or nucleophilic groups are prepared, it is desirable to separate the oligomers so that only higher molecular weight polymers are used as the first and second compounds. Oligomers can be conveniently separated by use of conventional dialysis techniques or ultrafiltration membranes. The polymers bearing onium and/or nucleophilic (or anionic) groups can optionally be derived from unsaturated moieties bearing other compatible groups. In some instances it may be desirable to use such compatible monomers in order to enhance certain properties of the resulting compounds, such as their hydrophobic or hydrophilic properties, their film-forming properties or glass transition temperature, For example, nonylphenoxy polyoxyethylene (10) methacrylate (9N10MA) or other surface active monomers can be used to render the polymer more wetting. Other compatible monomers include a C- | to C- ]2 alkyl methacrylate or hydroxyethyl methacrylate. Preferably, the first and second compounds display a good combination of properties. For example, it is desirable that said compound is soluble or dispersible in aqueous 5 media. At the same time the compound should be sufficiently wettable such that it can readily be deposited on the substrate on which the compounds are to be reacted. 10 In a preferred embodiment, the polymer used to form the discriminating layer is a terpolymer of hydroxyethylmethacrylate, vinylbenzyldimethyl sulfonium chloride and methacrylic acid. Other preferred polymers include vinylbenzyltrimethylammonium/methyacrylic acid 15 and vinylbenzyldimethylsulfonium/methyacrylic acid. As discussed above, it may be desired to use a separate solution to "wet" the support to facilitate interaction between the support and other coating 20 solutions. Those skilled in the art will recognize that various solutions are suitable for this. Examples of useful solutions include dilute alcohol solution and various polymer solutions. It should be noted that ?c - solutions should be avoided which may interfere with formation of the film. As discussed herein, the polymer solution useful in the formation of the discriminating layer may be used. 30 The affixing layer serves to affix the discriminating layer to the support. The discriminating layer is preferably physically affixed to the support. By affixed is meant that the discriminating layer is stabilized, i.e., unrestrained swelling of the layer is 35 prevented and delamination of the discriminating layer is substantially prevented. To accomplish this, the affixing layer polymer solution is preferably capable of insolubilization. Insolubilization may be accomplished, for example, by reaction with the solvent or via subsequent crosslinking and forming covalent bonds with residual reactive groups in the discriminating layer. It should be noted that in the alternative, or in conjunction with the affixing layer polymer solution, any unreacted portion of the discriminating layer polymer solution also serves to affix the discriminating layer to the support. By unreacted in this context, it is meant any portion of the discriminating layer polymer solution which does not form the discriminating layer at the air/blanketing layer interphase upon irradiation. Various polymers are suitable for use in the affixing layer polymer solution, so long as they serve this purpose and do not detrimentally affect the finished membrane. The polymers may be multi-component polymers. Multi-component polymers usually consist of different monomeric units each of which contributes a desired characteristic to the resulting polymer and ultimately to the finished membrane. For example, monomers may be used which contribute nucleophilic groups for reacting with or crosslinking through a cationic group, enhance the hydrophobic or hydrophilic properties of the membrane, exert a special affinity for the species which is to be separated using the finished membrane or adjust the mechanical properties of the resulting membrane. The polymer in the affixing polymer solution may be the same as or different from the polymer in the discriminating layer forming polymer solution and can be selected from those discussed herein in connection with the discriminating layer polymer solution. Other suitable polymers are well known to those skilled in the art and include those discussed in, for example, U. S. Patent 4,839,203 to Davis et al. issued June 13, 1989, relevant portions thereof hereby being incorporated by reference. It is preferred that the discriminating layer is formed by exposure to ultraviolet (UV) radiation, although other types of radiation may be used. 10 Generally, the radiation exposure should be at a combination of time and wavelength sufficient to form the desired film. Generally, a typical dose of UV radiation is from 0.01 to 20 joules/cm 2 although any dosage which result in the preparation of the membranes 15 of this invention is acceptable. If the dosage is too low, the film formed is too thin and thus lacks the necessary mechanical strength to be useful. If the dosage is too high, it 20 results in a thick, brittle film and results in a membrane having undesirably low flux. The discriminating layer of the membranes of this invention is generally very thin. The 25 discriminating layer is typically about 500 to 10,000 Angstroms. The MWCOs of the composite membranes of the present invention are altered by modifying the 30 conditions under which the membranes are produced. For example, membrane characteristics are influenced by adjusting the concentrations of polymer solutions used in the production of the membranes; modifying the n r - radiation dosage; varying the identity and concentration of additives used, and varying the affixing conditions. The properties of the composite membranes of this invention will vary with their MWCOs. Generally, membranes having a lower MWC0 will have pure water fluxes lower than those of higher MWC0, For example, a membrane having a MWC0 of 60 daltons or less generally should have a pure water flux of greater than 10 gallons per square foot of membrane per day (gfd) at 250 psi. Membranes with a MWC0 of about 200 daltons generally should have a pure water flux of greater than 25 gfd at 10 250 psi. Membrane devices of the spiral, tubular hollow fiber or plate and frame configuration can be fabricated from the membranes prepared as described herein. These 15 devices are assembled in accordance with conventional techniques once the membrane is prepared. The following examples are provided to illustrate the invention and should not be considered as 0 limiting its scope. Figures 1-8 show results obtained when membranes prepared by the process of this invention are tested. 5 Example 1 - Preparation of Membrane A polymerization initiator, 2,2'-azobis- isobutyronitrile was used to initiate free radical polymerization of 2-hydroxyethylmethacrylate, 30 vinylbenzyl chloride and methacrylic acid in tetrahydrofuran. After polymerization was complete, approximately 1.2 equivalents of dimethyl sulfide perequivalent of vinylbenzyl chloride was added to the - a ,- polymer solution. The solution was then heated to promote the conversion of the benzyl chloride moiety to benzylsulfonium chloride. Water was added the reaction mixture as necessary to keep the polymer soluble. The tetrahydrofuran and residual dimethyl sulfide were removed from the polymer solution under reduced pressure. The aqueous solution was then dialyzed against deionized water using dialysis tubing. The polymer solution was stored at 4°C until used. A polysulfone ultrafiltration membrane was made by casting a 15 weight percent solution of polysulfone in dimethyl formamide on a glass plate with 0.005 inch doctor blade and quickly immersing the plate in a water bath at room temperature. This membrane was used as the support in the comDosite membrane synthesis. An aqueous solution containing 0.1 weight percent of the polymer made as described above and 3 weight percent of H 2 S0i j was placed in a dish. A 2 inch in diameter disk of the polysulfone support was submerged under the solution which was maintained at room temperature. The solution was irradiated with a 450 W Ace-Hanovia 7825-34 UV lamp. The total dose of radiation delivered between 280 nm and 390 nm wavelength was 4 joules/cm 2 . The irradiation caused the formation of a thin polymer film at the air/solution interface. The film was loosened from the edge of the dish. The volume of the solution below the film was increased four times by the addition of water through a syringe. The submerged support was drawn up through the film to laminate the film on to the support which was then placed for one hour in an oven pre-heated to 90°C. Next, a 1.5 inch in diameter disk was cut out of the support. This disk was placed in a solution of 1:1 weight: eight isopropanol and water to rewet the polysulfone support. The disk was then immersed in deionized water for a few minutes and then assembled into a reverse osmosis test cell and tested sequentially with the following feed solutions: 2000 ppm NaCl in deionized water at 250 psi; 2000 ppm MgS04 in deionized water at 250 psi; 6 percent corn syrup in deionized water containing 50 ppm Thimerosal antimicrobial. Feed was circulated at a rate of 100 cm3/min. After steady state had been reached, permeate was collected. Flux of the permeate was determined by weighing the amount of permeate collected in a given time. The solute concentrations of the feeds and permeates were evaluated either by measuring the conductance of the solutions associated with the NaCl- or MgS04-containing feeds, or by high performance liquid chromatography (HPLC) of aliquots of the solutions associated with the corn syrup-containing feed. For the solutions associated with the NaCl or MgS04 feeds, conductance measurements were converted to absolute concentrations through calibrations. HPLC was conducted with an acetonitrile/water eluant with detection based on the index of refraction. The column separated the glucose oligomers present in the corn syrup. Oligomers containing from 1 to 13 glucose units were detectable. The amount of each oligomer present in the permeate solution was determined relative to that of feed solution by comparing the peak heights of the corresponding chromatograms. The percent solute rejection, SR, of the solute was calculated as for the NaCl- and MgS04-containing feeds as: [solute]permeate SR=100 x 1 - [solutejfeed and for the glucose oligomer with i glucose units, dpi, as SR=100 x(1-relative concentration of dpi in the permeate) The data on flux and solute rejection for the membrane are shown in Table I. The units of flux, gfd, are gallons/ft2-day. The limits of detection of the HPLC technique were such that when the rejection of a glucose oligomer was ≥98.5 percent, that glucose oligomer was not detectable in the permeate TABLE I Membrane Performance Example 2 In this example, the procedure outlined in Example 1 was followed with the exception that commercially available polysulfone ultrafilters available from FilmTec Corporation were used as the support and the area of the support was 250 cm 2 . The polymer concentration was 0.55 weight percent and the radiation dose was 2.5 J/cm : In each experiments (1-7), two to four samples are cut from each membrane. The results are shown in Table II. TABLE II Variation of Membrane Performance for Samples Made Under Identical Conditions 10 15 20 25 General Procedure for Membrane Preparation The general method, which will be the same for 30 all of the membranes synthesized in the following examples unless otherwise specified, was as follows: Approximately 12 cm diameter circles were cut from a sheet of machine-made polysulfone. These circles -,,- were stored in 1:1 isopropanol alcohol (IPA) IPA:H 2 θ until used. Prior to use, a circle was removed from the storage solution and immersed in approximately 11 of deionized (dl) dl H 2 O. After about 10 minutes, the dl H 2 O was placed with fresh dl H 2 O. The circles were then mounted on a special holder. The holder consisted of a 12 cm diameter disk of perforated stainless steel. Three posts were welded approximately equally spaced a few millimeters from the edges of the disk. A stainless steel ring about 0.5 cm wide with three holes to accommodate the posts was used to hold the polysulfone flat. After the polysulfone was mounted on the holder, it was immersed in dl H 2 O until it was used. A solution containing 0.1 weight percent of polymer and 3.0 weight percent H 2 SO was prepared. The polysulfone mounted in the holder was removed from the dl H2O. The holder and the polysulfone were blotted dry with a paper towel. The holder was then quickly placed in a 13.8 cm x 1.5 cm petri dish. 60 cc of the polymer solution was added. 60 cc was sufficient to immerse the polysulfone to a depth of a few millimeters. The petri dish containing the polymer solution and immersed polysulfone, at ambient temperature unless otherwise noted, was inserted into an UV irradiation chamber and irradiated, until 2.00 Joules/cm2 accumulated on the radiometer detector. The irradiation was then terminated, and the dish removed. The film was loosened from tne sides of the dish with a scalpel blade. The polysulfone holder was then lifted up through the interface at a shallow angle. When the holder was clear of the solution, it was positioned to be nearly vertical. The bottom side of the holder was then blotted on paper towel. The holder was then placed in an oven set at 100°C for 1 hour. During the last 5 minutes, a vacuum was drawn in the oven. The sample was removed from the oven and cooled. The polysulfone circle was removed from the holder. As many as three 3.0 cm diameter circles were punched from the polysulfone circle and assembled into test cells. The cells filled with a solution of 12.5 percent IPA in dl H 2 O. After about 15 minutes, the cells were flushed for >5 minutes with a stream of dl water. The cells were successively placed on lines circulating one of four different feeds: 2000 ppm NaCl; 300 ppm CaCl 2 ? 2000 ppm MgSθ4; or 10 percent glucose containing 50 ppm Thimerosal antimicrobial. The feed pressure was in all cases 130 psi. Example 3 Various membranes were prepared using a hydroxyethylmethacrylate/vinylbenzyl dimethylsulfoniom/methacrylic acid polymer and the General Procedure set forth above with the only variations being that no vacuum was pulled on the oven at the end of the curing period and the concentration of H2SO 4 used was varied as follows: 10 A c The results obtained when the membranes are tested using feed solutions containing NaCl, CaCl 2 , MgS0 4 and Glucose are shown in Figures 1-4. Example 4 0 The procedure in Example 3 was followed with the exception that the membranes were prepared by adding two weight percent of each of the following salts to the polymer solution: NaCl, LiCl, MgCl 2 ,MgS0 4 , Na 2 S0 4 , 25 Na 2 HP0 4 . The results are shown in Figures 5-8. Example 5 Various membranes were prepared using a hydroxyethylmethacrylate/vinylbenzyl 30 dimethylsulfoniom/methacrylic acid polymer and the General Procedure set forth above with the only the temperature at which irradiation occurred. The temperatures used were 0°C, 33°C and 55°C. 35 Example 6 In this example, the polymer used was poly(vinylbenzyltrimethylammonium c bicarbonate/methacrylic acid in the preparation of a membrane using the General Procedure set forth above. Example 7 Various membranes were prepared using a 10 hydroxyethylmethacrylate/vinylbenzyl dimethylsulfoniom/methacrylic acid polymer and the General Procedure set forth above with the only variation being the oven temperature at which the 15 membranes were cured after irradiation. The temperatures used were 70°C, 100° and 120°C. Example 8 Various membranes were prepared using the 20 General Procedure outlined above using the following polymers: 0.5 weight percent of a vinylbenzyltrimethylammonium/methyacrylic acid 25 (1:1) 0.5 weight percent of a vin lbenzyldimethylsulfonium/methyacrylic acid 30 (1:1) 0.1 weight percent of hydroxyethylmethacrylate/ inylbenzyldimethylsul fonium/methyacrylic acid (45:22.5:32.5) 35 0.1 weight percent of hydroxyethylraethacrylate/ inylbenzyldimethylsul fonium/methyacrylic acid (5:47.5:47.5) 1.0 weight percent of a vinylbenzyltrimethylammonium/methyacrylic acid (1:1) which received 4 J/cm 2 of radiation Example 9 The procedure set forth in Example 3 was followed using the additives and varying the ultraviolet radiation dose as shown in Table I below. In the last three entries in Table I, "NSA" refers to the sodium salt of 2-naphthalenesulfonic acid, a photosensitizer which was added to the polymers solution at 0.01 weight percent. Three samples were cut from each membrane and then tested as described in the General Procedure with the exception that no test was run using glucose. Flux is reported in units of gallons per square foot of membrane per day (gfd). TABLE I I > I N.A. = Not available

Claims: CLAIMS 1. A process for the formation of a composite membrane having a discriminating layer affixed to a porous support layer comprising the formation of the discriminating layer by irradiating a substantially aqueous polymer solution which comprises from 0.01 to 30 weight percent polymer and which is in contact with a blanketing fluid under conditions which are sufficient to form a polymer film on the surface of the polymer 10 solution at the blanketing fluid/solution interphase. 2. The process of Claim 1 comprising the following steps, performed simultaneously or sequentially: 15 ( 1 ) contacting a support with at least one polymer solution which is from 0.01 to 30 weight percent polymer in a substantially aqueous solvent; 0 (2) irradiating the polymer solution under conditions such that a discriminating layer is formed at the blanketing fluid/solution interphase; and 25 (3) affixing the discriminating layer to the support. 3. The process of Claim 1 comprising the 30 following steps, performed simultaneously or sequentially: (1) contacting a support with an affixing layer o c forming polymer solution which is from 0.01 to 10 weight percent polymer; (2) contacting the support with a discriminating layer forming polymer solution which is from 0.01 to 30 weight percent polymer in a substantially aqueous solvent; (3) irradiating the discriminating layer forming polymer solution under conditions such that a discriminating layer is formed at the blanketing fluid/solution interphase; and (4) affixing the discriminating layer to the support. 4. The process of Claim 1 wherein the blanketing fluid is air. 5. The process of Claim 1 wherein the polymer in the aqueous polymer solution comprises an onium group with an associated anion and a chromophore. 6. The process of Claim 5 wherein the discriminating layer forming polymer solution comprises a polymer selected from the group comprising hydroxyethylmethacrylate/vinylbenzyldimethyl sulfonium/methacrylic acid polymers; vinylbenzyltrimethylammonium/methyacylic acid polymers and vinylbenzyldimethylsulfonium/methyacylic acid polymers. 7. The process of Claim 1 wherein the substantially aqueous polymer solution further comprises an additive. 8. The process of Claim 7 wherein the additive is H 2 S0i. 9. The process of Claim 1 wherein the substantially aqueous polymer solution comprises at least fifty percent water.

Inventors: FIBIGER RICHARD F, FORGACH DAVID J, GLUGLA PAUL G, HARRIS J KEITH, SCHMIDT DONALD L, FIBIGER, RICHARD, F., FORGACH, DAVID, J., GLUGLA, PAUL, G., HARRIS, J., KEITH, SCHMIDT, DONALD, L.
Applicants: DOW CHEMICAL CO, FIBIGER RICHARD F, FORGACH DAVID J, GLUGLA PAUL G, HARRIS J KEITH, SCHMIDT DONALD L, FIBIGER, RICHARD, F., FORGACH, DAVID, J., GLUGLA, PAUL, G., HARRIS, J., KEITH, SCHMIDT, DONALD, L., THE DOW CHEMICAL COMPANY
Application year: 1994
Main classification: B01D67
IPC codes: B01D69, B01D71, C08L25, C08L33